What is Power sequence?

Question:

What is Power sequence?

Answer:

The power sequence is the sequence in which the power is applied and shut down, and includes specifications of the time intervals between steps in the sequence. A power sequence is used when there are semiconductor devices with different power supply voltages mixed together on one board, or when multiple equipment is connected. The time interval specifications differ depending on the device, which can cause problems when applying power to on-board devices with different power supplies.
In a general power sequence, power to the mainframe device is applied first, followed by the peripheral devices. The reverse occurs for power shutdown (peripheral devices first, mainframe last; however, the opposite may also apply, depending on the input signal functions). In other words, the power sequence is a procedure whereby the controlling side must fix the status of the controlled side.
There are not usually any problems with the power sequence in analog devices, but in digital devices such as MOS, the power sequence requires caution. This is because in MOS devices, the maximum input voltage value is generally prescribed by the absolute maximum ratings to be a value such as VDD + 0.3 V. Turning the power on in one device causes the rating of the connected device whose power is OFF (VDD = 0 V) to be exceeded when the output signal of the device that is ON becomes high level, leading to damage or degradation of characteristics. In this state, if the power supply of the device that was OFF is turned ON, a deadlock is also more likely to occur due to latch-up. Latch-up is another cause of characteristics degradation.
Note that neither of these problems occurs if the input buffer is a high-tolerance buffer with an absolute maximum input voltage of a value such as + 6.5 V. Problems also do not occur if the output from the output side is constantly a low level below the rated input voltage of the input side.

If the power sequence or circuit is defined by the interface specifications, this kind of problem can be solved. One example is the use of a hot-pluggable USB. When connecting an interface with an undefined power sequence, countermeasures can be taken such as making the output side open drain and pulling up or down the input side and making it a terminating resistor.
At this time, the output of the device whose power is OFF is floating, so through-current may flow through the input buffer of the device whose power is ON due to the effects of noise. The answer in this case is to pull this line down using a resistor.
If the input line is pulled up, the output of the device whose power is OFF will be damaged. If it is necessary to set the initial value of input of the device whose power is ON to high level, therefore, the output of the device whose power is OFF must be pulled up using a high-tolerance buffer that enables pull-up.

Note that voltage cannot be generated by an output that is open drain, so this countermeasure can also be used to adjust a high level on the line to the tolerant level of the input side when connecting devices with different power supply voltages.
If a CMOS output is used instead of an open drain, the device can be protected by flowing current to the power supply (0 V) that is OFF, by connecting a low-forward-voltage (VF) diode, such as a Schottky barrier diode, and a current limiting resistor. Moreover, if a capacitor is inserted between the signal line and the ground on the board as a countermeasure to noise, this circuit can also be used to protect against inrush current.

When the power of both devices is OFF, both the input and output configured by MOS FETs are high impedance, so the wiring becomes an antenna. In a noisy environment, therefore, this may cause the device's characteristics to degrade. The above countermeasures are effective in this kind of environment, even for a single-power-supply MOS digital circuit (the time constant of noise is considerably shorter than that of power application, so a diode is more effective than a resistor).
Of course, it is also necessary to take regular countermeasures to prevent noise during normal operation.

If the noise environment is fine, the devices can be electrically separated by using a MOS FET and switching the interface line using the power supply of the device to be switched on second.